Cellulose is a straight-chain polymer of anhydroglucose with β1-4 bonds. A great variety of natural materials comprise a high concentration of cellulose. Cellulose fibres in natural form comprise such material as cotton and hemp. Synthetic cellulose fibres comprise products such as rayon (or viscose) and a high strength fibre such as lyocell (marketed under the name TENCEL™).
Natural cellulose exists in either an amorphous or crystalline form. During the manufacture of synthetic cellulose fibres the cellulose is first transformed into amorphous cellulose. As the strength of the cellulose fibres is dependent upon the presence and the orientation of cellulose crystals, the cellulose material can then be re-crystallised during the coagulation process to form a material provided with a given proportion of crystallised cellulose. Such fibres still contain a high amount of amorphous cellulose. It would therefore be highly desirable to design a process to obtain cellulose-based fibres having a high content of crystallised cellulose.
Advantages of using cellulose for the manufacture of fibres includes its low cost, wide availability, biodegradability, biocompatibility, low toxicity, dimensional stability, high tensile strength, lightweight, durability, high hygroscopicity and easiness as to surface derivatization.
The crystallised form of cellulose which can be found in wood, together with other cellulose based material of natural origin, comprises high strength crystalline cellulose aggregates which contribute to the stiffness and strength of the natural material and are known as nano-fibres or nano-fibrils. These crystalline nano-fibrils have a high strength to weight ratio which is approximately twice that of Kevlar but, at present, the full strength potential is inaccessible unless these fibrils can be fused into much larger crystalline units. These nano-fibrils, when isolated from the plant or wood cell can have a high aspect ratio and can form lyotropic suspensions under the right conditions.
Song, W., Windle, A. (2005) “Isotropic-nematic phase transition of dispersions of multiwall carbon nanotube” published in Macromolecules, 38, 6181-6188 described the spinning of continuous fibres from a liquid crystal suspension of carbon nanotubes which readily form a nematic phase (long range orientational order along a single axis). The nematic structure permits good inter-particle bonding within the fibre. However natural cellulose nano-fibrils, once extracted from their natural material, generally form a chiral nematic phase (a periodically twisted nematic structure) when the concentration of nano-fibrils is above about 5-8% and would therefore prevent the nano-fibrils from completely orienting along the main axis of a spun fibre. Twists in the nano-fibril structure will lead to inherent defects in the fibre structure.
In the article “Effect of trace electrolyte on liquid crystal type of cellulose micro crystals”, Longmuir; (Letter); 17(15); 4493-4496, (2001), Araki, J. and Kuga, S. demonstrated that bacterial cellulose can form a nematic phase in a static suspension after around 7 days. However, this approach would not be practical for the manufacture of fibres on an industrial basis and is specifically related to bacterial cellulose which is difficult and costly to obtain.
Kimura et al (2005) “Magnetic alignment of the chiral nematic phase of a cellulose microfibril suspension” Langmuir 21, 2034-2037 reported the unwinding of the chiral twist in a cellulose nano-fibril suspension using a rotating magnetic field (5 T for 15 hours) to form a nematic like alignment. This process would not however be usable in practice to form a usable fibre on an industrial level.
Work by Qizhou et al (2006) “Transient rheological behaviour of lyotropic (acetyl)(ethyl) cellulose/m-cresol solutions, Cellulose 13:213-223, indicated that when shear forces are high enough, the cellulose nano-fibrils in suspension will orient along the shear direction. The chiral nematic structure changes to a flow-aligned nematic-like phase. However, it was noted that chiral nematic domains remain dispersed within the suspension. No mention was made relating to practical applications of the phenomena such as the formation of continuous fibres.
Work by Batchelor, G. (1971) “The stress generated in a non-dilute suspension of elongated particles in pure straining motion”, Journal of Fluid Mechanics, 46, 813-829, explored the use of extensional rheology to align a suspension of rod-like particles (in this case, glass fibres). It was shown that an increase in concentration, but especially an increase in aspect ratio of the rod-like particles results in an increase in elongational viscosity. No mention was made of the potential for unwinding chiral nematic structures present in liquid crystal suspensions. British patent GB1322723, filed in 1969 describes the manufacture of fibres using “fibrils”. The patent focuses primarily on inorganic fibrils such as silica and asbestos but a mention is made of microcrystalline cellulose as a possible, albeit hypothetical, alternative.
Microcrystalline cellulose is a much coarser particle size than the cellulose nano-fibrils. It typically consists of incompletely hydrolyzed cellulose taking the form of aggregates of nano-fibrils which do not readily form lyotropic suspensions. Microcrystalline cellulose is also usually manufactured using hydrochloric acid resulting in no surface charge on the nano-fibrils.
GB 1322723 generally describes that fibres can be spun from suspension which contains fibrils. However the suspensions used in GB 1322723 have a solids content of 3% or less. Such solids content is too low for any draw down to take place. Indeed, GB 1322723 teaches to add a substantial amount of thickener to the suspensions. It should be noted that the use of a thickener would prevent the formation of a lyotropic suspension and interfere with the interfibril hydrogen bonding that is desirable for achieving high fibre strength.
Also a 1-3% suspension of cellulose nano-fibrils, particularly one containing a thickener, would form an isotropic phase. GB 1322723 does not deals with the problems associated with using concentrated suspension of fibrils, and in particular using suspensions of fibrils which are lyotropic.